5.7. Dark Side of Packet Capture

What you can do, others can do. Pretty
much anything you can discover through packet capture can be
discovered by anyone else using packet capture in a similar manner.
Moreover, some technologies that were once thought to be immune to
packet capture, such as switches, are not as safe as once believed.

5.7.1. Switch Security

Switches are often cited as a way to
protect traffic from sniffing. And they really do provide some degree
of protection from casual sniffing. Unfortunately, there are several
ways to defeat the protection that switches provide.
First, many switches will operate as hubs,
forwarding traffic out on every port, whenever their address tables
are full. When first initialized, this is the default behavior until
the address table is built. Unfortunately, tools like
macof, part of the dsniff
suite of tools, will flood switches with MAC addresses overflowing a
switch's address table. If your switch is susceptible, all you
need to do to circumvent security is run the program.
Second, if two machines have the same MAC
address, some switches will forward traffic to both machines. So if
you want copies of traffic sent to a particular machine on your
switch, you can change the MAC address on your interface to match the
target devices' MAC address. This is easily done on many Unix
computers with the ifconfig command.
A third approach, sometimes called
ARP poisoning, is to send a forged ARP packet to
the source device. This can be done with a tool like
arpredirect, also part of
dsniff. The idea is to substitute the packet
capture device's MAC address for the destination's MAC
address. Traffic will be sent to a packet capture device, which can
then forward the traffic to its destination. Of course, the forged
ARP packets can be sent to any number of devices on the switch.
The result, with any of these
three techniques, is that traffic will be copied to a device that can
capture it. Not all switches are susceptible to all of these attacks.
Some switches provide various types of port security including static
ARP assignments. You can also use tools like
arpwatch to watch for suspicious activities on
your network. (arpwatch is described in Chapter 6, "Device Discovery and Mapping".) If sniffing is a concern, you may want to
investigate what options you have with your switches.
While these techniques could be used to routinely capture traffic as
part of normal management, the techniques previously suggested are
preferable. Flooding the address table can significantly degrade
network performance. Duplicating a MAC address will allow you to
watch traffic only to a single host. ARP poisoning is a lot of work
when monitoring more than one host and can introduce traffic delays.
Consequently, these aren't really techniques that you'll
want to use if you have a choice.

5.7.2. Protecting Yourself

Because of the potential for abuse,
you should be very circumspect about who has access to packet capture
tools. If you are operating in a Unix-only environment, you may have
some success in restricting access to capture programs. packet
capture programs should always be configured as privileged commands.
If you want to allow access to a group of users, the recommended
approach is to create an administrative group, restrict execution of
packet capture programs to that group, and give group membership only
to a small number of trusted individuals. This amounts to setting the
SUID bit for the program, but limiting execution to the owner and any
group members.
With some versions of Unix, you might
even consider recompiling the kernel so the packet capture software
can't be run on machines where it isn't needed. For
example, with FreeBSD, it is very straightforward to disable the
Berkeley packet filter in the kernel. (With older versions of
FreeBSD, you needed to explicitly enable it.) Another possibility is
to use interfaces that don't support promiscuous mode.
Unfortunately, these can be hard to find.
There is also software that can be
used to check to see if your interface is in promiscuous mode. You
can do this manually with the ifconfig command.
Look for PROMISC in the flags for the interface.
For example, here is the output for one interface in promiscuous
mode:

Of course, you'll want to check every interface.
Alternately, you could use a program like
cpm, check promiscuous mode
from CERT/CC. lsof, described in Chapter 11, "Miscellaneous Tools", can be used to look for large open files that
might be packet sniffer output. But if you have Microsoft Windows
computers on your network or allow user-controlled computers on your
network, this approach isn't enough.
While it may appear that packet
capture is a purely passive activity that is undetectable, this is
often not the case. There are several techniques and tools that can
be used to indicate packet capture or to test remote interfaces to
see if they are in promiscuous mode. One of the simplest techniques
is to turn your packet capture software on, ping an unused IP
address, and watch for DNS queries trying to resolve that IP address.
An unused address should be ignored. If someone is trying to resolve
the address, it is likely they have captured a packet.
Another possibility is the tool
antisniff from L0pht Heavy Industries. This is a
commercial tool, but a version is available for noncommercial uses.
There are subtle changes in the behavior of an interface when placed
in promiscuous mode. This tool is designed to look for those changes.
It can probe the systems on a network, examine their responses, and
usually determine which devices have an interface in promiscuous
mode.
Another approach is to restructure your
network for greater security. To the extent you can limit access to
traffic, you can reduce the packet capture. Use of virtual LANs can
help, but no approach is really foolproof. Ultimately, strong
encryption is your best bet. This won't stop sniffing, but it
will protect your data. Finally, it is always helpful to have clearly
defined policies. Make sure your users know that unauthorized packet
capture is not acceptable.